1. Field of the Invention
The present invention relates to a memory device including a semiconductor.
2. Description of the Related Art
There are many kinds of memory devices including semiconductors. For example, a dynamic random access memory (DRAM), a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), a flash memory, and the like can be given.
A DRAM stores data by holding electrical charge in a capacitor provided in a memory cell. However, at present, in a commercially available DRAM, even when a transistor used for switching is off, slight leakage current that is nonnegligible is generated between a source and a drain; thus, data is lost in a comparatively short time. Thus, it is necessary to rewrite (refresh) data at a constant frequency (generally every several tens of milliseconds).
An SRAM retains data by utilizing a bistable state of a flip-flop circuit. A CMOS inverter is generally used in a flip-flop circuit of an SRAM; however, six transistors are used in one memory cell and the integration degree of the SRAM is lower than that of a DRAM. In addition, the data is lost when power is not supplied.
On the other hand, in an EEPROM or a flash memory, a so-called floating gate is provided between a channel and a gate and electrical charge is stored in the floating gate, so that data is retained. In this specification, a memory having a floating gate, such as an EEPROM and a flash memory, is particularly called a floating gate nonvolatile memory (FGNVM). The electrical charge stored in the floating gate is held even after the supply of power to a transistor stops; thus, such a memory is called a nonvolatile memory. For example, Reference 1 may be referred to for a flash memory.
Since multilevel data can be stored in one memory cell in an FGNVM, storage capacity can be increased. Further, since the number of contact holes can be markedly decreased in a NAND flash memory, the integration degree can be increased to some extent.
However, an FGNVM needs high voltage at the time of injection of electrical charge to a floating gate or removal of the electrical charge. Thus, deterioration of a gate insulating film cannot be avoided and it is impossible to repeat write and erase operations without limitation.